Lipids constitute a broad group of naturally occurring molecules that include fats. In addition to fats, lipids may also include waxes, sterols, fat-soluble vitamins, monoglycerides, diglycerides, triglycerides, phospholipids, fatty acids, glycerophospholipids, sphingolipids, saccharolipids, polyketides, prenol lipids and sterol lipids, for example cholesterol. Lipids are vital components of cell membranes and have several forms and functions, are involved in many metabolic processes and are one of the major multifunctional agents present in human milk. Lipids also provide a form of energy storage and act as vehicles for absorption and transport of fat-soluble vitamins.
Fats are a subgroup of lipids generally referred to as triglycerides; they are a concentrated source of energy that can provide over 30% and up to 70% or more of dietary calories. Fat facilitates the absorption of fat-soluble vitamins and supplies essential fatty acids.
Milk, such as bovine milk, is a complex emulsion that contains several classes of components, including lipids and fats, which fulfill nutritional requirements and/or deliver special health benefits to the consumer. The fat component of milk exists in the form of globules which have a diameter which ranges in size from 0.1 to 20 micrometers. The presence of fat globules and the size and composition of the fat globules in milk contribute to the nutritional properties and other characteristics of milk.
The fat globules in milk comprise about 98% triacylglycerols (“TAGs”) and are stabilized by a cellular milk fat globule membrane (“MFGM”). Structurally, TAGs are derived from glycerol and include three fatty acid moieties. Upon digestion the fatty acids attached to the glycerol backbone are cleaved by digestional lipases and used by the body as nutrients. Accordingly, TAGs are the major storage form of energy in animals.
Milk may contain a variety of fatty acids, either as free fatty acids or as part of a TAG. For example milk fat may comprise saturated fatty acids, trans-fatty acids, monounsaturated fatty acids, polyunsaturated fatty acids, odd- and branched chain fatty acids (“OBCFAs”), branched chain fatty acids (“BCFAs”) and/or conjugated linoleic acid (“CLA”).
MFGM is the membrane surrounding the lipid droplets, which includes the fatty acids and TAGs, found in milk. MFGM consists of a complex mixture of phospholipids, proteins, glycoproteins, triglycerides, cholesterol, enzymes and other minor components. The chemical composition of MFGM is close to that of a cell membrane, typically having a bilayer composed of fatty acids and/or phospholipids. In bovine milk, the MFGM accounts for 2-6% of the mass of the milk fat globules.
One important property of milk fat globules is their size, both in terms of the average size of the total fat globules found in milk and the range or distribution of the fat globule sizes found in milk. In naturally occurring milk fat globules, the size of the globule can cause variations in the actual fatty acid composition of the triacyglcerols of the globule. For example, smaller milk fat globules contain more C18:0 and more C18:1 fatty acids than do larger milk fat globules.
Particularly with respect to human milk, the size of the fat globules vary with time postpartum, and are generally in the range of about 2.5 μm to about 5.0 μm, volume-surface average diameter, or from about 3.0 μm to about 6.0 μm, volumic average diameter. The specific surface area of human milk is, generally speaking, between about 1.0 m2/g and about 2.0 m2/g, and the mean free distance between fat globules in the human milk emulsion is understood to be between about 155 μm and about 195 μm. In the case of human milk, participation of milk proteins in the MFGM is minimal, with most milk proteins existing free in the emulsion, rather than forming an element of the MFGM.
During breastfeeding, the infant's suckling action produces lingual lipases in the mouth; these lingual lipases are active at a lower pH than pancreatic lipases. The phospholipid layer surrounding the human milk fat globules is relatively porous, and gets exposed to the lingual lipases in the stomach, which leads to release of free and monoacylglycerols of C8, C10, C12 and C14 fatty acids. These fatty acids have an antiviral and antibacterial effect, which help protect the infant. Moreover, the presence of lingual lipases facilitates the rapid digestion of fat in the infant's stomach, especially since bile salt dependent lipase (“BSDL”) and co-lipase dependent lipase (“CDL”), two of the other primary mechanisms of fat digestion in humans, are present at relatively low levels in infants.
While the size of fat globules in bovine milk is comparable to human milk, the situation changes when the bovine milk is homogenized. Homogenization of bovine milk can break the MFGM and/or increase the surface area of the globules by decreasing the fat globule size to less than 2 μm (volume-surface average diameter) or less than 3 μm (volumic average diameter). Casein micelles surround the membrane after homogenization and, when pasteurized, whey proteins are denatured and whey and casein surround the bovine milk fat globules, with MFGM components like phospholipids pushed to the aqueous medium.
With respect to commercially available pediatric nutritional compositions like infant formulas, many contain a lipid source from vegetable oils stabilized by added proteins and/or emulsifiers, with a globule size of less than 1.6 μm (volume-surface average diameter) or 2.2 μm (volumetric average diameter). The specific surface area of the infant formula fat globules is believed to be above 5.0 m2/g, and often significantly above 5.0 m2/g. These infant formula fat globules often have a dense cloud of denatured proteins surrounding the globule. Thus, the proteins need to be digested by gastric proteases before the globules can be accessed by lipases for lipid digestion. And, the relatively small globule size and higher surface area requires more proteases at a lower pH than larger globules would.
Moreover, the lipid source provided by vegetable oils lacks certain components of milk fat or milk fat globule membrane that are known to play an important role in pediatric and/or infant health and development. Replacing milk fat in nutritional compositions, such as infant formula, with vegetable oils may have other draw-backs, including non-reversible component interactions between proteins, lipids and minerals found in the nutritional compositions.
Therefore, pediatric subjects who consume infant formulas or pediatric nutritional compositions that have a fat source stabilized by added proteins may not be receiving adequate lipid nutrition.
Accordingly, it would be beneficial to provide a nutritional composition having an enriched lipid fraction that includes fat globules that are similar in size and composition to human milk fat globules. Additionally, since naturally occurring fat globule size can affect fatty acid composition of the milk fat globules, it would be beneficial to provide enriched lipid fractions that include milk fat globules of a desired size and fatty acid composition.
Further, it would be beneficial to provide milk fat globules that are stabilized by components similar to those found in the human milk, such as phospholipids, cholesterol and milk fat globule membrane proteins, instead of other added proteins and emulsifiers. Additionally, it is beneficial to provide a method of promoting digestion in a pediatric subject by providing a nutritional composition that contains a lipid source comprising an enriched lipid fraction having milk fat globules similar in chemical composition and size to those found in human milk.